The present invention relates to a method for the treatment of fly ash containing additives, and to the treated fly ash so obtained. In particular, the method relates to the grinding of fly ash containing additives and separation of magnetic particles therefrom. In preferred embodiments, the fly ash is subjected to treatment at high temperature prior to grinding e.g. to remove carbon products. An example of fly ash is Type F fly ash. The present invention also relates to so-called Ordinary Portland cement (OPC) containing fly ash and an additive, especially an additive that permits control and adjustment of the crystalline content of fly ash for optimization of performance of OPC containing fly ash.
Fly ash is a fine particulate product that is produced by the combustion of powdered coal with a forced draft, and is generally carried off with the flue gases. Under governmental and other regulations relating to the environment, it is necessary to remove fly ash from flue gases before discharge of the flue gases to the atmosphere. Special equipment is required to do so e.g. electrostatic precipitators.
The fly ash that is obtained is a mixture of alumina, silica, unburned carbon and various metallic oxides. There are indications that fly ash that passes through electrostatic precipitators in flue gas stacks may have mutagenic properties.
The metallic components of fly ash include aluminum, iron and titanium. These components may be recovered by calsintering i.e. heating the fly ash to 1200xc2x0 C. with a mixture of limestone and gypsum followed by treatment with sulphuric acid.
Fly ash is reported to have a wide variety of end uses, including as an additive for cement intended for oil-well casing, as an absorbent for oil spills, as a replacement for lime in the scrubbing of sulphur dioxide from flue gas, as a filler for plastics, for removal of heavy metals from industrial waste waters and separation of oil-sand tailings. However, as discussed below, addition of fly ash to ordinary Portland cement causes retardation of development of the strength of the cement. For example, addition of  greater than 20% of fly ash delays the setting of OPC and increases the period for attainment of the strength required to permit demoulding of the cement from a frame or mould. The setting may be delayed for many hours, the delay depending on the level of replacement of concrete with fly ash. The delay also depends on temperature. For example, in temperate climates, the setting could be delayed from a time of about 18 hours to a time of about 70 hours.
Further methods of treatment of fly ash, and further uses of treated fly ash, would be beneficial.
A method of treatment of fly ash, and the resultant product have now been found.
Accordingly, one aspect of the present invention provides a finely ground powder comprising a mixture of fly ash and at least one compound selected from the group consisting of alkali and alkaline earth metal salts, alkali and alkaline earth metal oxides, and metakaolinite.
In preferred embodiments of the invention, the amount of said compound is 0.4 to 1% by weight of the powder.
In a further embodiment, the powder has been treated for removal of magnetic particles.
In another preferred embodiment, the mixture has been subjected to a gas/solid reactor.
Another aspect of the present invention provides a method for the preparation of a composition comprising the steps of (a) admixing fly ash and at least one compound selected from the group consisting of alkali and alkaline earth metal salts, alkali and alkaline earth metal oxides, and metakaolinite, and (b) subjecting the mixture of fly ash and at least one compound to a fine grinding procedure.
In a preferred embodiment of the method of the present invention, the admixture of fly ash and at least one compound has been subjected to treatment in a gas/solid reactor. In particular, prior to step (b), the admixture of fly ash and at least one compound has been subjected to treatment in a gas/solid reactor e.g. to effect decarbonization of the fly ash. Preferably, the gas/solid reactor is operated under conditions that promote an increase in the amount of crystalline fraction of the fly ash relative to the amount of amorphous phase.
In embodiments, the admixture of fly ash and at least one compound so obtained is treated for removal of magnetic compounds.
In preferred embodiments, subsequent to step (b), the ground mixture is subjected to air classification to separate fine particles of a pre-determined particle size.
A further aspect of the present invention provides a composition comprising Portland cement and finely ground powder, said finely ground powder being a mixture of fly ash and at least one compound selected from the group consisting of alkali and alkaline earth metal salts, alkali and alkaline earth metal oxides, and metakaolinite. In particular, the amount of said compound is 0.4 to 1% by weight of the powder.
In preferred embodiments of the composition, the amount of said finely ground powder is at least 20% by weight of the composition, especially at least 25% by weight of the composition and preferably at least 30% by weight of the composition.